As described in the above-referenced '287 application, in a TDMA cellular communication system, a simplified illustration of which is diagrammatically shown in FIG. 1, communications between a base station BS and a desired user 11-1 in a centroid cell 11 are subject to potential interference by co-channel transmissions from users in cells dispersed relative to cell 11, particularly immediately adjacent cells, shown at 21-71. This potential for co-channel interference is due to the fact that the same frequency is assigned to multiple system users, who transmit during respectively different time slots.
In the non-limiting simplified example of FIG. 1, where each cell has a time division reuse allocation of three (a given channel is subdivided into three user time slots), preventing interference with communications between user 11-1 and its base station BS from each co-channel user in the surrounding cells 21-71 appears to be an ominous task--ostensibly requiring the placement of eighteen nulls in the directivity pattern of the antenna employed by the centroid cell's base station BS.
In accordance with the invention disclosed in the '287 application, this problem is remedied by determining times of occurrence of synchronization patterns of monitored co-channel transmissions from users in the adjacent cells, and using this timing information to periodically update a set of amplitude and phase weights used to control the directivity pattern of a phased array antenna. The array's antenna weights are updated as participants in the pool of interferers change (in a time division multiplexed manner), so as to maintain the desired user effectively free from co-channel interference sourced from any of the adjacent cells.
Since the maximum number of nulls than can be placed in the directivity pattern of a phased array antenna is only one less than the number of elements of the array, the fact that the number of TDMA co-channel interferers who may be transmitting at any given instant is a small fraction of the total number of potential co-channel interferers (e.g., six versus eighteen in the above example) allows the hardware complexity and cost of the base station's antenna to be considerably reduced. However, because the locations of co-channel interferers and therefore the placement of nulls is dynamic and spatially variable, the antenna directivity pattern must be controlled very accurately; in particular, excessive sidelobes that are created by grating effects customarily inherent in a phased array having a spatially periodic geometry must be avoided.